Mountain Ash

This work involved modifications to the weir at the Cynon River crossing on the former Deep Navigation colliery site. Named deep as at the time it was sunk between 1855 and 1860 it was the deepest in the coalfield. Picture 1, taken before the enhancement works, shows how difficult it was for migratory fish to move upstream; due mainly to its flat length. Only at very high river flows was it possible for Salmon and Trout to surmount this obstacle.

The required easement was effected in partnership with Natural Resources Wales (NRW) and used funds from the Taff and Ely Mitigation Fund. This Fund was established to improve migratory fisheries and is administered by NRW. SEWRT obtained the necessary land owner permissions to carry out the works. The easement necessitated the installation of timber baulks at intervals up the weir to create flows and depth that would allow migratory fish to pass over the weir at most water heights. The actual work was carried out by a local contractor, SEWRT’s favoured approach. Considerable care was necessary to avoid damage to a trunk sewer running along the East side of the weir; Dwr Cymru and Rhondda Cynon Taf (RCT) permissions were required. Pictures below show the works progress and outcome.

This weir was initially classed by NRW as passable only at high flows for Salmon and Trout. We now consider it to be passable at all flows. The improvement work at the site now makes a further 13.5km of river accessible; as far as the next remaining Cynon barrier at Station Road, Hirwaun. We are working with NRW and RCT to put a fish pass in place on that weir. Note in the pictures, the two new concentrated flows at the end of the weir. In high water and when fish are running these will be the two jumping points.

Weir prior to work being carried out the water above the weir is too shallow for salmon and trout to swim through

Work in progress

Finished project looking down stream, the flow concentrated in the middle